This invention relates to a method of storing information in optically readable form and to an optical data storage medium suitable for use in said method.
One class of data storage techniques comprises selectively irradiating the surface of a storage medium, the medium being such that, when irradiated at sufficient intensity and duration, its properties are altered in some manner which is readily detectable. In this way data can be written onto and read from the storage medium. The writing irradiation may be electromagnetic or particulate. The advantage of electromagnetic writing radiation is that the writing (recording) operation need not be carried out in vacuum, while that of particulate writing radiation is that for any specified resolution, or data packing density, the depth of focus is much greater than for electromagnetic writing radiation.
When electromagnetic writing radiation is used, one of the difficulties frequently encountered is that the proportion of the incident energy absorbed by the data recording and storage medium may be small, and the energy required to alter detectably its properties may be large. Either or both of these restrictions may increase the energy required to write a detectable data spot on the data storage medium, which can increase the cost of the radiator or slow down the writing speed, or both.
Strongly absorbent surfaces have been prepared by producing a surface which is rough on a microscopic scale and in which the depth of the texture is large compared with its pitch. Such a surface is described by Spiller et al in Applied Optics Vol. 19, No. 17, 1 Sept., 1980 pp 3022-3026. However, the methods so far known for the production of such surfaces, such as implantation and etching, are inherently expensive. Furthermore, surfaces made by etching techniques cannot easily be replicated because etch pits tend to include undercut areas and are often relatively deep.
European Patent Application No. 83305781.3 (published as a pending application under the publication No. 0107379 on May 2, 1984) discloses a data storage medium having a surface or a surface region which is strongly absorbent of at least a predetermined band of wavelengths of electromagnetic radiation, whereby the surface or surface region can be written-upon by such radiation, the said surface or surface region including a layer of heat sensitive material which has a textured surface pattern the pitch of which is smaller than the shortest wavelength within said predetermined band of wavelengths, and the depth (peak-to-trough) of which pattern is at least 100 nm, the textured surface pattern being free from undercutting so as to be suitable for production or replication by moulding, casting, embossing or similar process against a tool, and the textured surface pattern being overcoated with a thin film of a material which has a high opacity at the predetermined band of wavelengths. This will be referred to hereinafter as "the basic invention".
We have now developed the basic invention further, in that the present inventors have devised a new method of storing information in an optically readable form, and have further developed an optical data storage medium suitable for use in said method.
More particularly, in one aspect the invention provides a method of storing information in optically readable form, in which an illuminating beam of electromagnetic radiation of a predetermined wavelength band is directed at selected points on an optically sensitive recording medium in order to generate at said points areas of a first characteristic which can be differentiated on optical reading of the medium from areas thereof which have not been subject to illumination, characterised in that illumination is directed at an optical data storage member one surface of which is constituted by a material which undergoes a large volume change on degradation and which has a textured surface structure in the form of a regular pattern of sinuous form which is free from undercutting so as to be suitable for production or replication by molding, casting, embossing or similar process against a tool and having anti-reflection properties, and, overcoating said sinuous surface texture, and conforming thereto, a thin film of a metal; and in which the illumination is directed at said surface so as to cause local degradation of said material at the metal film/material interface whereby the metal film is deformed without rupture and is displaced from said material by products of said local degradation, whereby the anti-reflection properties of said surface are diminished or destroyed at the loci of illumination. The illuminating radiation can be monochromatic and is preferably coherent.
In a second aspect, the invention provides an optical data storage medium in the form of a substrate one surface of which is constituted by, or carries, a material which undergoes a large volume change on degradation and which has a textured surface in the form of a regular pattern of sinuous form which is free from undercutting so as to be suitable for production or replication by molding, casting, embossing or similar process against a tool and having anti-reflection properties, and, overcoating said sinuous surface texture, and conforming thereto, a thin film of a metal which comprises platnium or an alloy of platinum.